This invention is directed to a particle detection instrument and, more particularly, to a particle detection instrument with an improved detection cell for detecting particles in corrosive fluids.
Particle detecting instruments employ a detection cell which defines a fluid flow path through which a stream of fluid containing particles to be detected are caused to flow. The flow cell is provided with transparent walls so that a light beam can be transmitted through the flowing fluid. Particles passing through the light beam scatter light to a photodetector which generates pulses in response to the scattered light. The characteristics of the pulses, such as the pulse amplitudes, provide a measurement of the particle sizes. The detected scattered light may be forward scattered in the direction of the light beam or scattered at right angles to the light beam. Right-angle scattering geometry is preferred because it minimizes interference from stray light scattered from the entrance and exit windows of the cell. Right-angle scattering geometry requires the cell walls be transparent to the light all around the flow path. Early detecting cell designs with transparent walls surrounding the flow path utilize a polished round flow path. When a laser beam is focused to a narrow dimension illuminating the center of the flow path and suitable polish is present both on the outside and inside walls of the cell, the background scattered light from the cell will be sufficiently low to permit detection of scatter from particles as small as 0.1 microns in diameter. Round flow paths, however, have a drawback in that the laser beam cannot be broadened to illuminate a large area of the cell without increasing the background light to unacceptable levels. Accordingly, the view volume of such a cell is limited generally to less than 1 percent of the cell.
To overcome this problem, there has been developed a cell with a highly polished square flow path. This is accomplished with cells having quartz walls by making the cell out of four flat pieces which are highly polished and then diffusion bonding the polished pieces together. With this geometry, the laser beam can be widened to illuminate a much larger percentage of the flow path while still maintaining a low level of background scatter. With a quartz detection cell having a square flow path, the beam can be widened to cover 90 percent of the view volume and particles having a diameter of 0.2 microns can be detected. Particles having a diameter of 0.1 micron can be detected with a view volume of 4 percent of the flow path. Thus, the use of diffusion bonded quartz walls defining a rectangular flow path provides a highly efficient detecting cell.
However, quartz cannot be used to make up the walls of the detection cell if the fluid is corrosive and liquids containing particles to be measured are frequently too corrosive to pass through a quartz detection cell. When the fluid is corrosive, sapphire is the material of choice for the detection cell walls because it is not susceptible to being corroded by corrosive liquids. However, sapphire cannot be effectively diffusion bonded and, accordingly, the technique of providing a rectangular flow path as described above for the quartz cell cannot be used with sapphire.
There have been attempts to use ultrasonic drilling to cut a rectangular flow path through a sapphire prism to define a rectangular flow path in a sapphire detection cell. However, it is important for the flow path to have walls which are highly polished both on the inside and the outside and the rectangular flow path drilled through a sapphire prism can be polished with only one degree of motion, that is, parallel to the direction to the flow through the cell. Because the flow path can be polished with motion in only one direction, the level of polish achieved is inferior and causes the walls of the cell to scatter the light passing through the cell. In addition, the walls of the cell tends to collect particles from the flowing stream thus causing further background scatter. As a result, expensive imaging optics are utilized in the sapphire cell having the rectangular flow path drilled through the sapphire prism.
In contrast with the sapphire prism having a rectangular profile drilled therethrough, sapphire cells with round flow paths can have their inside walls polished to a satisfactory degree. This is because with a round flow path, the walls of the path can be polished with motion in two directions, axial and rotational. Even a round profile, however, cannot achieve the degree of polish that can be achieved on the quartz surfaces which are diffusion bonded together because the quartz surfaces can be polished with motion in several directions including rotary motion on the planes of the surface as well as linear motion in more than one direction on the planes of the surface. Accordingly, there is a need for a sapphire detection cell with a rectangular flow path which has the same high degree of polish that is achieved with the quartz detection cell having a rectangular flow path.